It has generally been known that, by combining a needle electrode serving as a discharge electrode and a plate electrode serving as an induction electrode and by applying a high voltage across the discharge electrode and the induction electrode, dielectric breakdown of air in the vicinity of a tip end portion of the needle electrode occurs and partial discharge takes place. This phenomenon is referred to as corona discharge.
An ion generation element utilizing this corona discharge phenomenon has been realized. Japanese Patent Laying-Open No. 10-199653 (Patent Document 1) discloses an exemplary electrode configuration generating negative ions as the ion generation element. Japanese Patent Laying-Open No. 10-199653 (Patent Document 1) describes the electrode configuration including a discharge electrode having a needle-shaped electrode to which a negative high voltage is applied, a perforated flat electrode provided opposed to the discharge electrode, to which a ground voltage or a positive high voltage is applied, and a cylindrical electrode attached to the perforated flat electrode.
In addition, Registered Japanese Utility Model No. 3028457 (Patent Document 2) also discloses an electrode configuration including a needle-shaped electrode. Registered Japanese Utility Model No. 3028457 (Patent Document 2) describes the electrode configuration including a needle-shaped corona generation electrode, a first opposing electrode in a cylindrical shape, and a second opposing electrode set up within the first opposing electrode, in which a tip end portion of the needle-shaped corona generation electrode is inserted in an opening at one end of the first opposing electrode in a cylindrical shape. In this electrode configuration, corona discharge occurs in the vicinity of the tip end of the needle-shaped electrode by applying a high voltage across the needle-shaped corona generation electrode and the cylindrical opposing electrode.    Patent Document 1: Japanese Patent Laying-Open No. 10-199653    Patent Document 2: Registered Japanese Utility Model No. 3028457